Disease relevance of Arnt

• These complexes regulate gene expression in response to hypoxia and xenobiotics, respectively, and mutation of the murine Arnt locus results in embryonic death by day 10.5 associated with placental, vascular, and hematopoietic defects [1].
• Inactivation of Arnt completely suppressed the development of hemangiomas, polycythemia, and Hif-induced gene expression [2].
• We cloned from mouse hepatoma cells a cDNA which encodes the Ah receptor nuclear translocator (Arnt) [3].
• The unique placental phenotype of Arnt(-/-) animals also provides an important tool for studying the disease of preeclampsia [4].
• The inhibitory domain displays specificity, in that it blocks the transactivating functions of AhR and Arnt, but not that of the herpes simplex protein VP16 [5].

High impact information on Arnt

• Together, these data suggest an important role for decreased ARNT and altered gene expression in the impaired islet function of human type 2 diabetes [6].
• Likewise, beta cell-specific ARNT knockout mice exhibited abnormal glucose tolerance, impaired insulin secretion, and changes in islet gene expression that mimicked those in human diabetic islets [6].
• Loss of ARNT/HIF1beta mediates altered gene expression and pancreatic-islet dysfunction in human type 2 diabetes [6].
• A heterodimer of the dioxin receptor (AhR) and Arnt, which are basic helix-loop-helix/PAS-family transcription factors, mediates most of the toxic effects of dioxins [7].
• By generating a targeted disruption of the Arnt locus in the mouse, we show here that Arnt-/- embryonic stem cells fail to activate genes that normally respond to low oxygen tension [8].

Chemical compound and disease context of Arnt

• Although Arnt is a necessary component of a functional nuclear AhR complex, this protein also forms transcriptionally active heterodimers with other bHLH/PAS factors, including those involved in the transcriptional response to hypoxia [9].
• Recruitment of thyroid hormone receptor/retinoblastoma-interacting protein 230 by the aryl hydrocarbon receptor nuclear translocator is required for the transcriptional response to both dioxin and hypoxia [10].
• Taken in sum, these experiments demonstrate that ARNT is an essential component of AHR developmental signaling and shed light on the mechanism of dioxin toxicity [11].
• Arnt-/- ES cells also failed to respond to a decrease in glucose concentration, indicating that ARNT is crucial in the response to hypoxia and to hypoglycaemia [8].
• Aryl hydrocarbon receptor regulation of ceramide-induced apoptosis in murine hepatoma 1c1c7 cells. A function independent of aryl hydrocarbon receptor nuclear translocator [12].

Biological context of Arnt

• Arnt contains a basic helix-loop-helix motif, which may be responsible for interacting with both the XRE and the ligand-binding subunit [13].
• 5. These results demonstrate that Arnt and Arnt2 have both unique and overlapping essential functions in embryonic development [1].
• Expression of Arnt2 successfully rescued XRE-driven reporter gene activity in the Arnt-defective c4 mutant of Hepa-1 cells [14].
• Each mutant was analyzed for dimerization with full-length mouse Arnt and subsequent binding of the dimer to the xenobiotic responsive element (XRE) [15].
• A 96-amino acid, C-terminal domain of Arnt, which includes a glutamine-rich region, confers transcriptional activation capability upon the protein [3].

Anatomical context of Arnt

• In addition, whole-mount in situ hybridization of 9.5-day mouse embryos showed that Arnt2 mRNA was expressed in the dorsal neural tube and branchial arch 1, while Arnt transcripts were detected broadly in various tissues of mesodermal and endodermal origins [14].
• Disruption of the Arnt gene in endothelial cells causes hepatic vascular defects and partial embryonic lethality in mice [16].
• Expression of MyoD in HeLa cells stimulated the Arnt promoter activity by binding to the E-box [17].
• Exon 6, encoding the conserved basic-helix-loop-helix domain of the protein, was flanked by loxP sites and introduced into the Arnt gene by standard gene disruption techniques using embryonic stem cells [18].
• Arnt concentration was not affected by TCDD in these cell lines [19].

Associations of Arnt with chemical compounds

• Gestational exposure of Ahr and Arnt hypomorphs to dioxin rescues vascular development [20].
• cDNA cloning and tissue-specific expression of a novel basic helix-loop-helix/PAS factor (Arnt2) with close sequence similarity to the aryl hydrocarbon receptor nuclear translocator (Arnt) [14].
• Western blot analysis of nuclei isolated from WT Hepa-1 cells fractionated with Nonidet P-40 shows that minimal levels of AhR or Arnt are retained in the nuclear fraction after TCDD treatment [21].
• Cobalt chloride induction of the glucose transporter-1 and heme oxygenase-1 mRNAs was also markedly abrogated in mice lacking ARNT expression, suggesting an inhibition of HIF-1alpha activity [18].
• Treatment with polyinosinic-polycytidylic acid to induce expression of Cre resulted in complete disruption of the Arnt gene and loss of ARNT messenger RNA (mRNA) expression in liver [18].

Physical interactions of Arnt

• MA10 cells also exhibit cAMP-dependent AhR down-regulation and AhR/Arnt complex formation [22].
• Treatment with the proteasome inhibitor MG132 increased endogenous Sp1 phosphorylation and its DNA-binding activity to the ARNT promoter [23].
• Formation of neural ARNT2/HIF-1alpha complexes in Arnt(-/-) ES cell-derived teratocarcinomas may explain why these tumors express VEGF, vascularize and grow efficiently, in contrast to ARNT-deficient hepatomas [24].
• The ligand-activated aromatic hydrocarbon receptor (AHR) dimerizes with the AHR nuclear translocator (ARNT) to form a functional complex that transactivates expression of the cytochrome P-450 CYP1A1 gene and other genes in the dioxin-inducible [Ah] gene battery [25].

Enzymatic interactions of Arnt

• In order to minimize any artificial steric hindrances to dimerization and XRE binding, each Ahr mutant was also tested with an equivalently deleted Arnt mutant [15].

Regulatory relationships of Arnt

• Genetic analyses reveal that induction is abolished in aromatic hydrocarbon receptor (AhR)- or aromatic hydrocarbon receptor nuclear translocator (Arnt)-defective variants but restored upon reconstitution of the variant cells with cDNAs expressing functional AhR or Arnt [26].
• Co-transfection of CHF1 inhibited ARNT/EPAS1-dependent transcription by 85%, and this inhibition is dose-dependent [27].

Other interactions of Arnt

• 5. These findings define the temporal regulation of receptor activation during normal ontogeny and provide evidence to support the idea that receptor activation and AHR-ARNT heterodimerization are essential for normal vascular development [20].
• Targeted mutation of the murine arylhydrocarbon receptor nuclear translocator 2 (Arnt2) gene reveals partial redundancy with Arnt [1].
• This places Ctsk approximately 4.5 kb downstream of Arnt on mouse chromosome 3 at locus 47.9 [28].
• Interaction of the regulatory domains of the murine Cyp1a1 gene with two DNA-binding proteins in addition to the Ah receptor and the Ah receptor nuclear translocator (ARNT) [29].
• BPA dose-independently up-regulated the expression of AhRR and Arnt in mid- and late-stage embryos [30].

Analytical, diagnostic and therapeutic context of Arnt

• Moreover, by using gel mobility shift assays we were able to show that the AhR/Arnt heterodimer binds to the XREs with very low affinity, which is due to three varied nucleotides outside the XRE core sequence [31].
• Detailed evaluation by immunoelectron microscopy of the AhR and Arnt present in the nuclear compartment of WT cells shows that both proteins are uniformly distributed and do not appear to be associated with nuclear pores, membranes, or nucleoli [21].
• In this study, we have elucidated the profile of genes responsive to the reintroduction of Arnt expression in an Arnt-deficient mouse hepatoma cell line (c4), using DNA microarray analysis [32].
• The Arnt cDNAs of c4 cells were cloned by reverse-transcription/PCR to compare the sequences with that of wild-type Arnt cDNA [33].
• The aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator protein show distinct subcellular localizations in Hepa 1c1c7 cells by immunofluorescence microscopy [21].

References